Tall and slim buildings and installations are subjected to particular vibration conditions which have to be taken into consideration by technical measures in order that no damage or premature fatigue processes occur. This applies, in particular, to wind turbines, which, owing to their rapid technical development in recent years, are increasingly also being employed in more extreme locations (e.g. offshore) and heights, and in addition have ever-higher towers in order to utilise the better wind conditions there. Such wind turbines have to withstand the forces acting on them due to wind, waves, weather and operation, which load the installations to different extents at different places. Vibration forces in particular can endanger the operation and safety of the installations. It is thus necessary to damp the vibrations occurring in these installations specifically and effectively by technical measures. This is carried out using vibration absorbers or vibration dampers of various design in accordance with the diverse different fields of use.
Thus, there are, for example, installations in which critical vibrations only represent a problem due to so-called Karmann vortex excitation. Since the rotating rotor generally effects very good damping of the wind turbine, vortexes of this type only occur at standstill. Owing to the damping of the rotor blades, which are transverse to the wind at standstill, an installation of this type can only be excited in the longitudinal direction. Since the Karmann vortex excitation takes place at 90° to the wind direction, this case only occurs in the transverse direction, more precisely only if the installation does not follow the wind direction. In installations of this type, it is therefore sufficient if a vibration absorber only works at standstill of the installation.
Of course, wind turbines also vibrate transverse to the rotor axis in operation. In this direction, the damping by the rotor is very small, so that, in particular in the case of very tall towers, additional damping by vibration absorbers in the transverse direction is also necessary.
Furthermore, there are installations which are constructed in the sea and are excited to vibration in all directions by wave excitation. During operation, such installations, owing to the rotor which is only damped in the axial direction by wind, react, in particular, to waves moved perpendicular to the wind direction. Vibration absorbers which act transversely to the rotor axis are therefore also necessary there during operation of the installation. Owing to the fact that such installations are also excited in all directions at standstill, vibration absorbers which act in the rotor direction and transversely to the rotor direction are also required from case to case.
Furthermore, there are excitations which are excited by the rotation of the rotor. These are on the one hand excitations by the rotor on passing through the resonant frequency of the tower. Towers are likewise excited by rotor frequencies which are close to the resonant frequency of the tower.
Thus, there are a number of different tasks for vibration absorbers in accordance with the excitation frequencies, some may be mentioned here: (i) vibration absorbers in the axial direction at standstill, (ii) vibration absorbers in the transverse direction at standstill and during operation, (iii) vibration absorbers in the horizontal plane at standstill, and (iv) vibration absorbers in the horizontal plane at standstill and during operation. Most interfering forces also act on the tower of the installation, which is generally excited to vibration with a low resonant frequency usually <1 Hz. For this reason, pendulum absorbers, which are usually suspended in the tower, are often employed in the prior art for vibration damping.
Pendulum absorbers for wind turbines are described, for example, in EP 1008747 (B1) or in WO 2009/068599. Classical pendulums are used here, but these can only move over small swing distances owing to the small space available for movement in the tower, which has the consequence that the absorber masses required must be fairly large in order to be able to exert an influence on the corresponding resonant frequency. In order to counter this, space-saving solutions have been proposed.
Thus, JP 08-200438 (A) discloses a vibration absorber which comprises an absorber mass which is fitted with rollers and is able to roll to and fro thereon on a circular rail arrangement which is curved in a concave manner towards the inside, where the mass moves de facto like the mass of a virtual pendulum with a pendulum rod or pendulum cable corresponding to the circle radius. Whereas the coarse adjustment to the resonant frequency is defined by the coarse configuration of the system (mass, rail dimensions, length, et cetera), the fine tuning of the resonant frequency in this system can only be carried out by changing the rail curvature, which is technically very complex and in some cases not possible at all.
EP 2746483 (A1) presents a roller absorber, likewise based on a pendulum movement, for a wind turbine, in which an optionally variable absorber mass can be moved out of a central position on a wheel/rail arrangement by restoring forces, in particular spring forces. The absorber mass/rail arrangement is arranged here within a surrounding framework, where the proposed height adjustment in this frame provides the desired fine tuning of the frequency, since the height adjustment causes a physical change in the length of the virtual pendulum. Apart from the fact that the entire device is relatively bulky and should still cause space problems in some areas of the wind turbine, the change or adaptation of the frequency by raising or lowering the heavy absorber mass, which can be about 500-5000 kg in the case of a wind turbine, is, however, quite difficult to achieve.
The object is therefore to provide a vibration absorber, in particular for damping resonant frequencies below 5 HZ, in particular below 1 Hz, in wind turbines or similar tall and slim installations or buildings, which is suitable for the restricted available space and meets the requirements of simple adjustability and adaptability of the resonant frequency to the local and operational conditions of an installation of this type taking into account the relatively heavy absorber masses required.
The object has been achieved by the vibration absorber described below. It has been found here that the vibration absorber according to the invention can be made available in various embodiments owing to its general novel concept, enabling most of the low-frequency vibration events, as summarised briefly above, occurring in a wind turbine or similar installation to be successfully damped or eliminated entirely.